This inventions relates generally to fluorescent lamp electrodes, and more particularly to a coiled filament electrode for use in high vibration environments.
Fluorescent tube lamps are used in many applications due to their desirable characteristics. A fluorescent lamp generally comprises a tube having a filament electrode at either end with the interior of the tube coated with a fluorescent material. The filament electrode usually comprises a coil of relatively thin wire. The coil is attached to a lead-in wire to which electrical contact is made with the filament electrode. The filament electrode being made of a coil or relatively thin wire is relatively delicate. The use of multiple coils additionally makes the filament electrode relatively delicate and often difficult to attach to a support or lead-in wire. The use of additional supports may add weight to the filament electrode which is disadvantageous, especially in high vibration environments. Accordingly, in high vibration environments, such as applications in avionics, premature lamp failure often results due to breakage of the filament electrode where it is attached to a mount or support. Additionally, it has been determined that there is a large variation in lamp life in a high vibration environment. Some lamps may fail relatively quickly, while other lamps have a much longer life. Therefore, it is often very difficult to predict lamp life. This results in unpredictable failures, or additional cost to provide more frequent maintenance replacements to avoid an unexpected failure. This is particularly disadvantageous in applications where a lamp failure may be critical, such as in avionics.
Additionally, filaments are often made of tungsten which is a refractory metal and in standard practice, is never melted but rather is made into wire by a combination of staging and drawing. As a result, tungsten is a metal with a pronounced grainy structure and gives tungsten a characteristic brittleness or tendency for small micro-cracks to propagate along the grain boundaries if subject to stress may be caused by vibration. Typically, multi-coiled tungsten filaments must be secured to lead-in connectors by either clamping or welding. If clamped, the tungsten filament must be clamped with sufficient force to provide good electrical contact and ensure that the filament does not come loose. However, the clamping must not be excessive so as to severely damage the crystal structure by inducing severe grain boundary cracks. In standard usage the clamping need be only minimally sufficient to survive stresses induced by shipping. Once installed in a low vibration environment, such as a home or office, it can be expected to perform well. Therefore, clamping variations in a low vibration environment is seldom critical. However, when subjected to severe vibration and shock, such as in avionics, the clamping becomes critical. A slightly under clamped filament can easily become loose under vibration, while an over clamped filament can induce micro cracks in the grain which will propagate along the grain boundaries resulting in a break at the filamentxe2x80x94clamp juncture. There is currently no nondestructive test that can determine if the clamping is adequate.
Therefore, there is a need to improve the vibration resistance or robustness of filament electrodes used in fluorescent lamps. There is also a need to improve the consistency of lamp life so that any lamp failure can be readily predicted, providing for improved and more reliable maintenance schedules.
The present invention is directed to a robust fluorescent lamp filament electrode suitable for high vibration environments. In a coiled filament attached to mounting posts, a mandrel is placed within each end of the coiled filament where it attaches to the mounting post. A portion of the mounting post is bent and crimped over each end of the coiled filament with the mandrel placed therein. A portion of the mandrel extends inward towards the portion of the coiled filament between the two mounting posts. Vibrations in the coiled filament between the two mounting posts are effectively dampened, reducing metal fatigue where the coiled filament is attached to the mounting posts. Additionally, the mandrel helps to secure the coiled filament in the mounting posts. The mandrels may be formed in the coiled filament by winding a tungsten wire around a molybdenum mandrel which is chemically dissolved except for end leg sections which are clamped.
Accordingly, it is an object of the present invention to improve the life of a fluorescent lamp having a coiled filament electrode.
It is another object of the present invention to improve consistency of the life of the lamp.
It is an advantage of the present invention that it is relatively easily manufactured.
It is a further advantage of the present invention that existing coiled filament electrodes can be used.
It is a feature of the present invention that a mandrel is placed or allowed to remain within each end of the coiled filament electrode where the electrode is attached to a mounting post.
These and other objects, advantages, and features will become readily apparent in view of the following more detailed description.